Conventionally, proteins directly extracted from living body components have been often used as blocking agents used for immunoassays. In particular historically, albumin and casein derived from bovine have been widely used. However, recently, various limitations have occurred owing to problems such as mad cow disease. Meanwhile, method of producing them using recombinant technology has an advantage that a pathogen (substance) can be excluded, but is virtually off from practical use due to problems such as productivity.
Accordingly, it can be said that an attempt to use a protein derived from Escherichia coli as an alternative protein is preferable in terms of productivity. However, there has been a problem in that a protein capable of being expressed on a large scale does not always have a good blocking efficiency and a protein which can be produced by recombinant technology and is excellent in blocking efficiency must be found.
Recently, gene sequences in various organisms have been elucidated, but a principle to find a protein excellent in blocking efficiency by a predicted amino acid sequence has not be known yet, and it has seemed that it takes a lot of trouble to search such a protein.
Due to such reasons, an alternative protein which can be produced on a large scale by recombinant technology and is excellent in blocking efficiency has been required.